Crossed loop antennas with separating shield

ABSTRACT

An omnidirectional antenna system having two loop antennas disposed at right angles to each other and electrically and magnetically decoupled by a shield between the antennas.

United States Patent Rinaldo E. De Cola Park Ridge;

Alicja D. Kulbiej, Chicago, both 01, ll]. 77 1,577

Oct. 29, 1968 July 20, 1971 Warwick Electronics lnc.

lnventors A pp]. No. Filed Patented Assignee CROSSED LOOP ANTENNAS WITHSEPARATING SHIELD 11 Claims, 5 Drawing Figs.

Int. Cl Olq 7/08, l-lOlq 1/52 Field of Search 343/742,

[56] References Cited UNITED STATES PATENTS 3,031,667 4/1962 Wennerbcrg.343/788 3,051,903 8/1962 Morrow 343/701 3,440,542 4/1969 Gautney 343/7883,447,159 5/1969 Stromswold 343/742 Primary Examiner-Eli LiebermanAttorney-Hofgren, Wegner, Allen, Stellman and McCord ABSTRACT: Anomnidirectional antenna system having two loop antennas disposed atright angles to each other and electn'cally and magnetically decoupledby a shield between the antennas.

CROSSED LOOP ANTENNAS WITH SEPARATING SHIELD This invention is concernedwith an omnidirectional antenna system. More particularly, two loopantennas, bidirectional in nature and disposed at right angles to eachother, are decoupled by conductive shielding located between the twoantennas.

tennas an equal amount above and below the center frequen- In all knownsystems of this type, however, it has been found that between theantennas themselves, there is a degree of coupling with is frequencydependent. This coupling produces a phase shift over the band offrequencies to be received with a resulting degradation in theomnidirectivity of the system. This coupling, as a practical matter, maybe minimized by precise 90 orientation of the antennas, however, evenwith accurate alignment, it is impossible to eliminate all couplingsbetween the antennas. Additionally, it should be pointed out thataccurate orientation of the antennas is very difficult and also costlyfrom a manufacturing standpoint.

The present invention provides an omnidirectional antenna system inwhich the coupling between the 90 displaced loop antennas is effectivelyeliminated whereby substantial omnidirectivity is achieved over anextremely wide band of frequencies while at the same time the cost ofthe system is reduced by eliminating the extreme accuracy required inthe 90 displacement of the antenna.

In accordance with the present invention, two loop antennas are disposedat substantially right angles to one another on opposite sides of agrounded conductive shield which substantially eliminates all couplingtherebetween. The physical separation and 90 orientation of the antennasis also effective to enhance the decoupling thereof.

Since the conductive shield effectively decouples the antennas, thesystem may be rendered omnidirectional over a wide band of frequenciesby employing critical remote coupling, stagger tuning the antennas orany other suitable method of obtaining the necessary quadrature relationbetween the signals in the two antennas.

While achieving the improved wide band omnidirectivity the presentinvention also reduces the cost of such system by virtue of the factthat the conductive shield effectively decouples the antennas toeliminate the critical 90 displacement factor which is required in theabsence of the shield to obtain maximum decoupling.

The present invention may be more fully understood by reference to thefollowing detailed specification and the drawings, in which:

FIG. 1 is a schematic diagram a preferred embodiment of the antennasystem of the present invention.

FIG. 2 is a schematic diagram of another embodiment of the antennasystem of this invention.

FIG. 3 illustrates the frequency response of the individual loopantennas shown in FIG. 2.

FIG. 4 depicts the physical relationship of the two antennas.

FIG. 5 illustrates the shield used in isolating the antennas.

The invention is illustrated herein as it is used in an amplitudemodulated (AM) radio receiver circuit. However, the invention can beused in a transmitter circuit and with coupling circuits other thanthose specifically shown and described.

Each loop antenna in this system is similarly constructed so that thegain and other characteristics of the antennas are identical. Anomnidirectional antenna pattern is achieved by the superposition of twoequal gain patterns upon one another. Two bidirectional, i.e. figureeight, patterns having a common center point and displaced at rightangles to each other provide a circular, omnidirectional pattern uponthe vector addition of the patterns.

FIG. illustrates a preferred embodiment of the present invention whereinthe loop antennas l0 and 12 are remotely critically coupled to producesignals in phase quadrature relation. Each of the antennas 10 and 12 areshunted by a variable ganged capacitor 14 and 116 respectively and havetheir-low impedance ends coupled to ground through a common inductor 20.The antennas are physically separated and oriented at to each other, asshown in FIG. 41. An electrically conductive shield 18 (shown in dottedline in FIG. 1) is located between the antennas and is grounded at 115.A coil 22 forms a transformer secondary for antenna 12. One end of coil22 is grounded and the other end is coupled to the input of RF amplifier23. Coil 22 picks up the signals in antenna 12 which are then amplifiedin RF amplifier 23 from which the signals are coupled to conventionalreceiving apparatus (not shown).

Each antenna of the omnidirectional system comprises a number of turnsof wire would on a rod or cylindrical core member 56 of a magneticferrite material so that the combined inductance of the antenna isapproximately 490 microhenries. The ferrite rod provides a lowreluctance path for the magnetic field, minimizing stray fields outsidethe ferrite and forming a high Q inductance. A ferrite rod antenna byitself is very directional. The antenna coil extends over substantiallythe entire length of the core 56 and is tightly wound thereon. Eachantenna has a circular cross section and is supported on grounded shield18 by L-shaped brackets 58, as shown in FIG. 1. The antennas are placedat 90 to each other, such that centerlines, through, and perpendicularto, the respective circular cross sections, are skewed and substantiallyat 90 to one another. Shield R8 is composed of laminated parallel'layersof a conductor, as a metal 60, and paper 61, as shown in FIG. 5.

A pair of single loop antennas, if disposed perpendicularly to eachother are effectively decoupled only if they are disposed symmetricallyabout a common center point. Since it is impossible to so dispose theferrite rod antennas shown in FIG. 4, the 90 orientation cannotcompletely decouple the antennas. Additionally, the plural elongatedloops of such antennas produced even further coupling problems. Bydisposing a grounded conductive shield such as shield 18 between theantennas, however, complete decoupling is achieved and the attendantproblems of coupling are eliminated.

in order to derive the necessary phase quadrature relationship betweenthe signals received by each antenna, the inductor 20 is provided.Inductor 20 is selected with regard to the Q of the antenna circuitssuch that critical coupling is achieved. This critical coupling, bydefinition, causes a phase quadrature component of the signal from oneantenna to appear in the Other. Thus, secondary or takeoff winding 22couples to the RF amplifier both the signal received by antenna 12 andthe signal received by antenna 10 shifted 90 i.e., the signals are inthe required phase quadrature relationship, and a completelyomnidirectional system is achieved. This system is essentially free ofany frequency dependent coupling between the antennas, and the 90orientation of the antennas is rendered less critical since the effectthereof on directivity is less than the degradation caused by couplingbetween the antennas in nonisolated systems wherein slight variations inthe 90 orientation causes a substantial increase in the coupling.

Although a single, common inductor 20 is shown in FIG. 1, it is to beunderstood that separate critically coupled inductors connected inseries with each antenna could also be used.

FIG. 2 illustrates another embodiment of the present invention whereinthe phase quadrature relationship between the antenna signals isachieved by stagger tuning the antennas.

Antennas 10' and 12' are the same as in FIG. 1, and are shunted byvariable ganged capacitors l4 and 16' respectively. The antennas arephysically separated, again as in FIG. 4. The conductive shield 18 islocated between the antennas as in FIG. 1. Output coils l9 and 21 arecoupled to the antennas l and 12' respectively and have one endconnected to ground. Coils l9 and 21 form transformer secondaries forantennas l0 and I2 and couple the signals received thereby to the inputbases 24 and 26 of RF transistor amplifiers 28 and 30. The emitters 32and 34 of the PNP transistors are connected together as are therespective collectors 36 and 38. A combined output signal is developedacross load resistor 40 connected form ground to the commoncollectorjunction.

In order that the system be omnidirectional, the signals at the baseinputs 24 and 26 must be in phase quadrature. In this embodiment of theinvention the phase quadrature relation is achieved by stagger tuningthe resonant circuits of each anterina in the manner shown in thefrequency response curves of FIG. 3. The resonant circuit comprised ofcapacitor 14 and the inductance of antenna is tuned to a frequencyfbelow the center frequency f., as indicated by curve A. The resonantcircuit comprised of capacitor 16' and the inductance of antenna 12' istuned to a frequencyf which is above f by an amount equal to thedifference between f and f See curve B.

By appropriately tuning the resonant circuits in this manner the signalsappearing in takeoff coils l9 and 21 will have the requisite phasequadrature relationship and the system will be omnidirectional. As inthe system shown in FIG. 1, the complete decoupling of the antennas bythe use of the shield 18 eliminates the degradation caused by phasevariations due to coupling between antennas and thus betteromnidirectivity is achieved.

We claim:

I. An omnidirectional antenna system comprising:

conductive shield means;

first and second loop antennas having a plurality of turns,

said antennas having bidirectional signal receiving patterns; said firstand second loop antennas being disposed on opposite sides of saidconductive shield means and positioned at right angles to each othersuch that the center turn of each antenna fomi; a crosslike projectionin the plane ofsaid conductive shield means, said conductive shieldmeans functioning to decouple said loop an tennas from each other; and

circuit means for combining signals leceived by said two antennas.

2. The omnidirectional antenna system of claim I wherein the antennasare substantially equal distances from the conductive shield.

3. The omnidirectional antenna system of claim 1 including means fortuning each antenna to resonance.

4. The omnidirectional antenna system of claim 3 wherein the firstantenna is tuned to a frequency above the resonant frequency of thesystem and the second antenna is tuned to a frequency below said systemresonant frequency, the stagger tuned antenna having frequencies oftiming equally separated above and below the system resonant frequency.

5. The omnidirectional antenna system of claim 4 including means forphysically connecting each antenna to the conductive shield.

6. The omnidirectional antenna system of claim 4 wherein the loops aresymmetrical providing similar frequency response curves when theantennas are tuned to their resonant frequency.

7. The omnidirectional antenna system of claim 1 wherein eachbidirectional antenna has an equal gain.

8. The apparatus of claim 1 wherein said circuit means comprises, meansfor establishing a phase quadrature relation between the signalsreceived by said antennas and means for combining the phase-quadraturerelated signals.

9. The apparatus of claim 8 wherein said means for establishing saidphase quadrature relation comprises means for remotely, criticallycouplin said antennas. 10. The apparatus of claim wherein said means forcritically coupling said antennas comprises a common inductor connectedin series with both of said antennas.

11. The apparatus of claim 9 wherein said means for combining saidphase-quadrature related signals comprises a secondary winding coupledto one of said antennas and amplifier means coupled to said secondarywinding.

1. An omnidirectional antenna system comprising: conductive shieldmeans; first and second loop antennas having a plurality of turns, saidantennas having bidirectional signal receiving patterns; said first andsecond loop antennas being disposed on opposite sides of said conductiveshield means and positioned at right angles to each other such that thecenter turn of each antenna forms a crosslike projection in the plane ofsaid conductive shield means, said conductive shield means functioningto decouple said loop antennas from each other; and circuit means forcombining signals received by said two antennas.
 2. The omnidirectionalantenna system of claim 1 wherein the antennas are substantially equaldistances from the conductive shield.
 3. The omnidirectional antennasystem of claim 1 including means for tuning each antenna to resoNance.4. The omnidirectional antenna system of claim 3 wherein the firstantenna is tuned to a frequency above the resonant frequency of thesystem and the second antenna is tuned to a frequency below said systemresonant frequency, the stagger tuned antenna having frequencies oftuning equally separated above and below the system resonant frequency.5. The omnidirectional antenna system of claim 4 including means forphysically connecting each antenna to the conductive shield.
 6. Theomnidirectional antenna system of claim 4 wherein the loops aresymmetrical providing similar frequency response curves when theantennas are tuned to their resonant frequency.
 7. The omnidirectionalantenna system of claim 1 wherein each bidirectional antenna has anequal gain.
 8. The apparatus of claim 1 wherein said circuit meanscomprises, means for establishing a phase quadrature relation betweenthe signals received by said antennas and means for combining thephase-quadrature related signals.
 9. The apparatus of claim 8 whereinsaid means for establishing said phase quadrature relation comprisesmeans for remotely, critically coupling said antennas.
 10. The apparatusof claim 9 wherein said means for critically coupling said antennascomprises a common inductor connected in series with both of saidantennas.
 11. The apparatus of claim 9 wherein said means for combiningsaid phase-quadrature related signals comprises a secondary windingcoupled to one of said antennas and amplifier means coupled to saidsecondary winding.